A re-writable card colors when it is heated above certain temperatures and de-colors when it is re-heated below the coloring temperature. FIG. 19 shows an example of the print media's coloring & de-coloring and the temperature. Since coloring starts at temperatures above T4 (180° C.), characters or images can be printed by heating these records above temperature T4 and cooling quickly down to temperature T1 (80° C.). The printed records can be erased (de-colored) completely by re-heating to temperature range between T2 (120° C.) and T3 (165° C.). In this case, there may be some residual image in the temperature ranged between T1 (80° C.) and T2 (120° C.). If the temperature goes up between T3 (165° C.) and T4 (180° C.), then re-coloring will start. Therefore, it is extremely important to maintain precise temperature control in printing (coloring) and erasing (de-coloring) processes. Additionally, the temperature between the T0 (25° C.) and T1 (80° C.) shown on FIG. 19 is a non-reactive region and there is no change in coloring regardless the way the media is heated or cooled. When the temperature goes beyond T5 (about 200° C.), the heated location becomes degraded and discolored which results in it being impossible to de-color. Those temperatures are for one exemplary recording media and actual temperatures will differ from media to media.
This type of printing and erasing on re-writable cards is done by a re-writable card printer shown in FIG. 20 as an example. More specifically, the process is carried out as follows:
The re-writable card RC is inserted in the card slot 51. The card goes through the print head 53 and the erase head 54 via the carrier route 52 which is made of multiple rollers 52a, 52b and 52c. The re-writable card stops at the hold location P and the switch for the erase head 54 power is turned on to start heating up while standing by. The card movement direction is reversed to go through between the erase head 54 and erase head support platen roller 54a for erasing process when the erase head temperature reaches at the predetermined level. The card RC comes out of the slot 51 after the printing is erased. When the card RC needs to be re-printed, it is inserted again in the card slot 51 and it goes between the print head 53 and print head support platen roller 53a while the new information is printed by the print head 53. When erasing and printing are performed continuously, it has to be done after the temperature of the card RC is cooled below T1 on the FIG. 19.
The erase head 54 is used for the equipment, for example, is shown graphically as the top and side views on FIG. 21. The heat element 56 is connected to the electric power on the ceramic substrate 55 surface and the thermistor 57 is attached on the back side of the ceramic substrate 55 in order to detect if the temperature reached to erasing temperature or not. The ceramic substrate 55 is held with the holder 58 which is made of the material such as plastic. When the thermistor 57 is placed on the back side of the ceramic substrate 55 to detect if the temperature has reached to the predetermined level or not, the heat element 56 has to generate large amount of heat to accommodate the high heat capacity of the ceramic substrate 55. This will take a long time to make the device erase-ready, 15 seconds for example.
The invention uses a heat element with large temperature coefficient to detect the temperature through the change of current through the heat element. The actual erasing occurs when the card and heat element are in contact, so achieving the erasing temperature at the point is adequate and erasing can be achieved regardless of the temperature of the back side of the ceramic substrate and the response time can be much faster.